Set.prototype.forEach() - JavaScript | MDN (original) (raw)

Baseline

Widely available

The forEach() method of Set instances executes a provided function once for each value in this set, in insertion order.

Try it

function logSetElements(value1, value2, set) {
  console.log(`s[${value1}] = ${value2}`);
}

new Set(["foo", "bar", undefined]).forEach(logSetElements);

// Expected output: "s[foo] = foo"
// Expected output: "s[bar] = bar"
// Expected output: "s[undefined] = undefined"

Syntax

forEach(callbackFn)
forEach(callbackFn, thisArg)

Parameters

callback

A function to execute for each entry in the set. The function is called with the following arguments:

value

Value of each iteration.

key

Key of each iteration. This is always the same as value.

set

The set being iterated.

thisArg Optional

A value to use as this when executing callbackFn.

Return value

Description

The forEach() method executes the providedcallback once for each value which actually exists in theSet object. It is not invoked for values which have been deleted. However, it is executed for values which are present but have the value undefined.

callback is invoked with three arguments:

• the element value
• the element key
• the Set object being traversed

There are no keys in Set objects, however, so the first two arguments are both values contained in the Set. This is to make it consistent with other forEach() methods for Map and Array.

If a thisArg parameter is provided to forEach(), it will be passed to callback when invoked, for use as itsthis value. Otherwise, the value undefined will be passed for use as its this value. The this value ultimately observable bycallback is determined according tothe usual rules for determining the this seen by a function.

Each value is visited once, except in the case when it was deleted and re-added beforeforEach() has finished. callback is not invoked for values deleted before being visited. New values added before forEach() has finished will be visited.

forEach() executes the callback function once for each element in the Set object; it does not return a value.

Examples

Logging the contents of a Set object

The following code logs a line for each element in a Set object:

function logSetElements(value1, value2, set) {
  console.log(`s[${value1}] = ${value2}`);
}

new Set(["foo", "bar", undefined]).forEach(logSetElements);

// Logs:
// "s[foo] = foo"
// "s[bar] = bar"
// "s[undefined] = undefined"

Specifications

Browser compatibility

See also